In order to improve fuel economy, a fuel cut-off control for stopping fuel supply to an engine during running, and a coasting control for coasting the vehicle by disconnecting the engine from a drive line during running have been employed in recent years. Specifically, the fuel cut-off is carried out to stop fuel supply to the engine given that an accelerator is returned to close an accelerator valve completely, and that an engine speed is higher than an idling speed so that a vehicle speed is higher than a predetermined speed. Under the fuel cut-off control, although the combustion of fuel in the engine is stopped, the engine is still rotating by a torque from the drive wheels. Therefore, a braking torque resulting from a pumping loss of the engine and a friction torque is applied to the drive wheels. Consequently, an engine braking force is applied to the vehicle.
For example, the coasting control is carried out when the accelerator is returned during running to coast the vehicle without rotating the engine passively by bringing the clutch into disengagement disposed between the engine and the drive wheels. Therefore, under the coasting control, the engine braking force will not be applied to the vehicle so that the vehicle is allowed to coast utilizing an inertia energy.
The coasting control may be carried out not only by stopping the fuel supply to the engine but also without stopping the fuel supply to the engine while lowering the engine speed close to the idling speed. In case of carrying out the coasting control while stopping the engine, the fuel will not be consumed during coasting the vehicle so that the fuel economy can be improved significantly. By contrast, in case of carrying out the coasting control without stopping the engine, the fuel economy will not be improved as the case of stopping the engine. In this case, however, it is not necessary to arrange additional devices for insuring a required oil pressure under cessation of the engine such as an electric oil pump, a hydraulic accumulator etc. Therefore, the coasting control can be carried out easily without requiring additional modification of the vehicle and additional equipment. In the following explanation, the coasting control without stopping the engine will be called the “neutral coasting control” or the “N coasting control”.
For example, Japanese Patent Laid-Open No. 2011-117497 describes a controller for a vehicle driving device configured to carry out the coasting control. The controller taught by Japanese Patent Laid-Open No. 2011-117497 is applied to a vehicle driving device comprised of an engine and an electromagnetic clutch disposed, on a power transmission route between the engine and drive wheels to selectively provide a connection therebetween. According to the teachings of Japanese Patent Laid-Open No. 2011-117497, a power transmission between the engine and the drive wheels is cut-off by the electromagnetic clutch given that an engine braking force exceeds a driving force propelling the vehicle established by operating an accelerator. Japanese Patent Laid-Open No. 2011-117497 further describes a fact that the engine and the drive wheels is connected by increasing a torque transmission capacity of the electromagnetic clutch at a predetermined ratio with respect to an elapsed time, given that a brake is depressed during interrupting the power transmission therebetween.
Japanese Patent Laid-Open No. 2002-227885 describes a clutch control device comprised of a clutch disposed on a power transmission route between an engine and drive wheels, and a control valve for actuating the clutch. The control device is configured to allow the vehicle to coast by opening the clutch by the control valve. According to the teachings of Japanese Patent Laid-Open No. 2002-227885, the coasting mode is cancelled if a brake pedal is depressed by a pressure greater than a predetermined pressure, if a distance from a forerunning vehicle shorter than a predetermined distance, or if a vehicle speed exceeds an upper limit speed or falls below a lower limit speed. Japanese Patent Laid-Open No. 2002-227885 further discloses to adjust an engine speed during the coasting mode in a manner such that a speed difference of the clutch is reduced to be lower than a predetermined value, and to temporarily interrupt such control for reducing the speed difference of the clutch when cancelling the coasting mode.
Japanese Patent Laid-Open No. 2010-60010 describes a control device for automatic transmission. According to the teachings of Japanese Patent Laid-Open No. 2010-60010, a power transmission between an engine and drive wheels is selectively interrupted by a clutch, and a vehicle speed is maintained to a constant speed during running down a downslope. When the vehicle runs down a slope where a downgrade is smaller than a set value, a sliding fastening control of the clutch is executed without altering a speed ratio. In this case, if the downgrade of the slope exceeds the set value, the vehicle speed is maintained to a constant speed.
Japanese Patent Laid-Open No. 2011-144878 describes a hydraulic control device for vehicular transmission comprising a hydraulic engagement device. The hydraulic engagement device is adapted to transmit an engine power to drive wheels when engaged, and provided with a cushion plate as a spring member interposed between a frictional engagement element and a pushing portion of the piston. In order to reduce an idling load of the engine, the hydraulic control device executes a neutral control upon satisfaction of a predetermined condition, by causing the engagement device to slip or bringing the engagement device into disengagement thereby restricting a power transmission between the engine and the drive wheels. Japanese Patent Laid-Open No. 2011-144878 further discloses to temporarily hold an engagement, pressure of the engagement device when cancelling the coasting control at a standby pressure to be applied to the cushion plate to push the cushion plate.
The controller taught by Japanese Patent Laid-Open No. 2011-117497 is configured to connect the power transmission route between the engine and the drive wheels to terminate the coasting upon activation of the brake device during coasting the vehicle while disconnecting the power transmission route. According to the teachings of Japanese Patent Laid-Open No. 2011-117497, therefore, the vehicle is decelerated not only by a braking force generated by depressing a brake pedal but also by an engine braking force resulting from a pumping loss and a friction torque. For this reason, the vehicle can be decelerated promptly in line with the driver's intension.
However, fuel economy may be reduced if the coasting is terminated based on an operation of the brake device during the coasting as taught by Japanese Patent Laid-Open No. 2011-117497. Basically, the deceleration demanded by the driver by depressing the accelerator pedal differs depending on a vehicle speed. Therefore, if the coasting control is terminated based only on an operation of the brake device, the coasting control may be terminated in spite of the fact that the driver does not intends to decelerate the vehicle significantly. In this case, fuel economy to be achieved by the coasting control would be reduced. In addition, the vehicle may be decelerated more than expected and the vehicle has to be accelerated unnecessarily. Consequently, the fuel economy of the vehicle would be further reduced.